I went on a life aboard to the Similan Islands in November with West Coast Divers in patong beach. Very friendly and professional staff. We where with 4 customers on a boat that could handle 16 customers. The owner Milton said he didn't make any money on this trip but felt it was his duty that if people would fly around the world for a trip like this he had the obligation to take them out. So making profit is not his main goal and to me that is a good point.
Have fun.
http://www.westcoastdivers.com/index.htm